Fan cover having a non-stick and/or non-wetting coating, cooking appliance comprising such a fan cover and method for manufacturing a fan cover

ABSTRACT

The present invention relates to fan cover (6), in particular a fan cover (6) for a cooking appliance (1), obtainable by a process characterised by the following steps, a) providing a fan cover (6) having an inner surface (7a), an outer surface (7b) and at least one opening (8) allowing a fluid to flow through, b) pretreating of at least the inner surface (7a) of the fan cover (6) for providing a surface having a roughness being suitable for applying a non-stick and/or non-wetting coating (12) by mechanical treatment, physical treatment or chemical treatment, in particular by sandblasting and/or laser treatment and/or an enamelling process to form a ground layer (13), c) applying the non-stick and/or non-wetting coating (12) to the inner surface (7a) of the fan cover (6) or the surface (14a) of the ground layer (13), wherein the non-stick and/or non-wetting coating (12) comprises at least one layer (17) that is obtained by a sol-gel process from a first composition comprising a silica sol and a silane. The invention further relates to a cooking appliance (1), in particular a domestic oven comprising such a fan cover (6) and a method for manufacturing such a fan cover (6).

This application claims priority benefit to European patent application No. EP19209417.5, filed on Nov. 15, 2019, and International patent application No. PCT/CN2020/083446, filed on Apr. 7, 2020, the disclosures of which are hereby incorporated by reference herein in their entirety.

TECHNICAL FIELD

The present invention relates to a fan cover having a non-stick and/or non-wetting coating. In particular, the present invention relates to a fan cover of a cooking appliance. Further, the present invention relates to a cooking appliance having such a fan cover. Moreover, the present invention relates to a method for manufacturing a fan cover, in particular a fan cover of a cooking appliance, wherein the non-stick and/or non-wetting coating is applied to the fan cover.

BACKGROUND

One problem for the customer is the cleaning of an oven after usage, especially after food items, such as cheese, sauces, oils, fat or the like, are burned onto the surface of the cavity and further components within the cavity, in particular a fan cover being attached to a rear wall of the cavity and covering the fan being provided at or within the rear cavity wall. Therefore, there are different approaches on the market to make cleaning easier. On the one hand, so-called self-cleaning appliances with pyrolysis functionality or appliances with catalytic enamel are known already. On the other hand, there are appliances with special enamel that is more easy to clean and less sensitive to impacts and scratches, e.g. granite enamel. Furthermore, appliances with plasma treatment or steam-cleaning program to improve the cleaning procedure and the cleaning result are already on the market.

Said actual systems have different disadvantages. Appliances with pyrolysis functionality require additional components for the pyrolysis system, what leads to higher costs. Furthermore, high temperatures are needed for the cleaning process, what costs energy. Appliances with catalytic enamel require high burning temperatures of over 800° C. during its manufacturing process. Further, the catalytic enamel has a brittle surface what may be problematic during assembly. Furthermore, over time the pores of the enamel get blocked and the cleaning effect worsens.

SUMMARY

It is an object of the present invention to provide a fan cover, a cooking appliance comprising such a fan cover and a method for manufacturing such a fan cover, having improved properties, in particular with regard to cleaning behaviour, temperature resistance and/or surface hardness.

These and other problems are solved by the subject matter of the attached independent claims.

The first object of the present invention is achieved by a fan cover according to claim 1.

A fan cover (or: fan shield), in particular a fan cover for a cooking appliance, obtainable by (or: prepared by) a process characterised by the following steps:

a) providing a fan cover, in particular an essentially plate-shaped fan cover, having an inner surface, an outer surface and at least one opening allowing a fluid to flow through,

b) preferably, pretreating of at least the inner surface of the fan cover for providing a surface having a roughness being suitable for applying a non-stick and/or non-wetting coating, by mechanical treatment, physical treatment or chemical treatment, in particular by sandblasting and/or laser treatment and/or a surface activation treatment, particularly a plasma treatment, and/or an enamelling process to form a ground layer, thus applying an enamel layer onto the inner surface of the fan cover,

c) applying the non-stick and/or non-wetting coating, directly or indirectly, to the pretreated inner surface of the fan cover, wherein the non-stick and/or non-wetting coating comprises at least one layer that is obtained by a sol-gel process from a first composition comprising a silica sol and a silane.

In other words: The at least one layer, hereinafter also referred to as base layer, is composed of a matrix comprising the condensation reaction product of a silica sol and a silane. Obtained by a sol-gel process from a first composition has therefore to be understood in such a way, that the first layer is obtained by means of hydrolysis and (poly-)condensation of an aqueous mixture comprising at least a silica sol and a silane. Thus, the starting materials of the first composition, namely at least silica sol, silane and water if needed, are mixed together and stirred. The mixture obtained thereof is then applied to the inner surface of the fan cover and dried to obtain the first layer. Such a coating having one layer is known for example from EP 2 177 580 B1.

The expression “non-stick” indicates a surface that resists adherence of food items, such as cheese, sauces, oils, fat or the like, what permits easy cleaning of the fan cover. The expression “non-wetting” terms a surface that repels liquids, e.g. water. The property is evidenced by a large contact angle between a drop of water and the surface on which the drop rests, wherein a contact angle of at least 95 degree is desirable.

According to the present invention, however, the “inner surface” of a fan cover is the surface directed to a heated or heatable cooking chamber defined by the cavity walls of a cooking appliance. Accordingly, an “outer surface” of a fan cover is the surface facing away from said cooking chamber.

The idea of the present invention is to provide a fan cover with an easy to clean effect whereby the contact angle of the surface shows more than 95 degrees. Applying a non-stick and non-wetting coating by a sol-gel process has the advantage that a burning process at high temperatures for applying a non-stick coating on the fan cover is not required what saves energy. Besides a good cleaning behaviour, said coating shows good abrasion-resistant and a great surface hardness what leads to an increased lifetime.

The adhesion of the non-stick and/or non-wetting coating to the pretreated fan cover of stainless steel or having a protective layer may particularly depend on the surface roughness. The present inventors have surprisingly found that such pretreatment before applying the non-stick and/or non-wetting coating and particularly a roughening of the surface is advantageous for the adhesion properties of the non-stick and/or non-wetting coating. Without such pretreatment, particularly such roughening, the at least one layer, and any subsequent layer, of the non-stick and/or non-wetting coating may peel off. Therefore an advantageous surface structure may reduce the risk of peel off of the non-stick and/or non-wetting coating and at the same time allow to reduce the amount of coating and thus decrease costs. However, a surface roughness, which is too high, may also be disadvantageous, as the adhesion of the coating may be not optimal and/or the amount of coating needed will be higher than necessary.

A pretreatment of the surface by laser may be particularly advantages in comparison to known sandblasting methods in that integration into other production equipment is possible, for example related to existing laser welding lines in oven cavities, where sandblasting equipment may need totally new equipment and specific housings, protections, or the like. Furthermore it is also advantageous in having no need of process material, like corundum powder usually applied in sandblasting according to the prior art. Furthermore, it is also advantageous in reducing dirt occurring during the treatment and further surface cleaning after pretreatment steps and/or before coating is advantageously reduced compared to standard sandblasting methods. Also reproducibility of surface parameter values are advantageously increased.

The person skilled in the art will recognize that various laser methods and laser equipment may be applied to achieve the described preferably surface roughness. By way of example, in a pretreatment step according to the present invention a laser of type CL100 can be applied with a focal width of 254 mm, a wave length of 1064 nm, and an optical system STAMP10 with suction removal. Thereby, the surface treatment speed may be adjusted according to the desired results and with methods well known to the person skilled in the art. For example such surface treatment speed may be approximately 10 m²/sec or higher.

According to the invention, the surface of the fan cover might be roughened and/or protected against corrosion by applying a ground layer by an enamelling process, thus by applying an enamel layer, to the surface. The enamelling process in particular comprises applying an enamel powder onto the inner surface of the fan cover and/or onto the surface of the protective layer, and heating the enamel powder for providing a ground layer with a surface having a roughness being suitable for applying the non-stick and/or non-wetting coating and/or being corrosion resistant.

Such a ground layer is useful for all base materials in order to prepare the fan cover for subsequent coating, thus for providing a good wettability and clean surface to obtain good adhesion performances on the full surface and no detachment of the non-stick and/or non-wetting coating.

Furthermore, said ground layer may provide corrosion resistance for the surface of the fan cover so that it also functions as a protective layer, in case of a fan cover made of a non-corrosive material itself. Therefore roughening the inner surface of the fan cover by an enamelling process is advantageous with regard to mechanical treatment like sandblasting or by laser treatment as just one single process step has to be carried out in order to form surface that has a good roughness and is in particular additionally non-corrosive in comparison to apply a protective layer in a first step and to roughen the surface of the protective layer in a second step. [Please check]

The fan cover can be made of different materials, the material being suitable for intended use, such as stainless steel or another material that is non-corrosive itself, for example aluminium or an aluminium alloy. The disadvantage of such fan covers is, however, that they are expensive. Therefore, in a preferred alternative, the fan cover is made of corrosive steel, particularly so-called low-carbon steel is a preferred material for the fan cover. An example for such commonly used low carbon steel typically contains a small percentage of carbon, approximately about 0.05 to 0.30% carbon.

In case of a fan cover made of corrosive material, the fan cover is preferably provided with (or: comprises) a protective layer (or: non-corrosive layer) that is applied to at least the inner surface of the fan cover and said protective layer is pretreated by sandblasting and/or laser treatment and/or an enamelling process, thus applying an enamel layer for providing a surface having a roughness being suitable for applying the non-stick and/or non-wetting coating and the at least one layer of the non-stick and/or non-wetting coating is applied to said protective layer.

For example, the surface of the fan cover is coated with such a protective layer by spray-painting.

This is due to the reason, that the non-stick and/or non-wetting coating itself does not protect the base material or substrate, thus the fan cover, from corrosion. Therefore, an additional corrosion protection layer is advantageous if the fan cover is made of any corrosive material so that the non-stick and/or non-wetting coating can also be used with base material or substrate that is corrosive itself. In case the base material is non-corrosive itself, the step of applying a protective layer may be optional.

According to preferred embodiments, said protective layer is an enamel layer and/or an aluminium layer and/or a layer comprising aluminium. In the case that the fan cover is manufactured from stainless steel or another non-corrosive material, such a protective layer may, but not necessarily has to be applied for corrosion protection.

Preferably the protective layer has a thickness between 50 and 250 μm, in particular between 100 and 200 μm.

The protective layer and/or the ground layer might be made of a so-called pyro or vitreous enamel that essentially consists of melted and fused glass powder. For applying said enamel layer to the fan cover, high burning temperatures from about 820 to 840° C. are needed.

Another kind of protective layer and/or the ground layer might be made of an enamel having a low softening point between 450° C. and 560° C., respectively a low glass transition temperature, a so-called “low-temperature enamel”. The softening point is defined as a temperature, respectively a temperature range, at which the material, e.g. the enamel turns from its hard and relatively brittle state into a molten or rubber-like state when temperature is increased. Therefore, said low-temperature enamel can be applied to the surface of the fan cover at lower burning temperatures. Said enamel is for example an aluminium enamel that comprises aluminium oxide.

Compositions of such conventional enamels are known for example from WO 02/02471 A1 or GB 718,132. WO 02/02471 A1 discloses an enamel composition for use in forming an enamel cover coat comprising a glass component comprising at least a first glass frit, comprising by weight from about 30% to about 45% P₂O₅, from about 20% to about 40% Al₂O₃ from about 15% to about 35% Li₂O and Na₂O and K₂O, up to about 15% B₂O₃, up to about 15% MgO and CaO and SrO and BaO and ZnO, up to about 10% TiO₂ and ZrO₂, and up to about 10% SiO₂. The glass component may also comprise a blend of at least a first glass frit and a second glass frit. GB 718,132 discloses alkali metal aluminium phosphate vitreous enamel fits.

Another kind of protective layer and/or the ground layer might be made of a catalytic enamel, for example known from EP 0 565 941 A1, disclosing a catalytically active coating composition comprising an enamel frit, inert substances and an oxidation catalyst, or from GB 1 418 842 disclosing a catalytically active enamel layer for a cooking appliances.

Furthermore, so-called ground coat compositions comprising generally a glassy component and an additive component might be used for forming the protective layer and/or the ground layer that provides the effect of a rough and binding surface. Such ground coats are for example described in U.S. Pat. No. 8,778,455 B2 and comprise a combination of one or more alkali oxides, one or more alkaline earth oxides and one or more various transition metal oxides as glassy component, and and additive such as fluorine.

According to a preferred embodiment, the ground layer, in particular the enamel layer obtainable by the enamelling process, has a layer thickness between 5 and 100 μm, in particular between 10 and 80 μm, more particularly between 20 and 50 μm. Typical enamel thickness is normally larger than 100 μm. Inventors have surprisingly found, that the enamel compositions mentioned above allow forming of thinner layers. In particular, in the case of a fan cover made of stainless steel, it is not needed that said thin enamel layer forms a continuous layer to improve wetting and adhesion properties of the surface of the fan cover as just a non-continuous layer already gives a favorable rough surface. An enamel that is too thin to form a continuous layer already gives a favorable rough surface.

[Please Check Whether all of the Above Mentioned Materials are Used for Both, the Protective Layer and the Ground Layer (Having Non-Corrosive Properties) Applied by Special Enameling Process]

For providing good adhesion properties of the non-stick and/or non-wetting coating, the surface is pretreated to provide a surface of the fan cover that has roughness being suitable for applying the non-stick and/or non-wetting coating. In a preferred embodiment a surface roughness of a surface of the fan cover and/or a surface of the ground layer and/or a surface of the protective layer achieved with such pretreatment step is between Ra 0.01 μm to 10.00 μm, more preferably between Ra 0.10 μm to 5.00 μm, still more preferably between Ra 0.20 μm to 5.00 μm, still more preferably between Ra 0.50 μm to 5.00 μm, still more preferably between Ra 2.00 μm to 5.00 μm, still more preferably between Ra 2.50 μm to 5.00 μm.

Such an advantageous surface structure with a roughness within the above limits may further reduces the risk of peel off of the non-stick and/or non-wetting coating and at the same time allow to reduce the amount of coating and thus decrease costs in a favorite manner. The present inventors have surprisingly found that a roughness within the above limits further improves the adhesion of subsequent layers or of a first or any subsequent layers of the non-stick and/or non-wetting coating to the fan cover or the protective layer and/or the ground layer.

In summary, a surface having such a good roughness can be obtained in particular by applying a ground layer of known enamels as mentioned before. Even mechanical, physical or chemical treatment, in particular laser treatment of the surface can be carried out to achieve required roughness.

A lower surface roughness—within the limits above mentioned—may be advantageous as the amount of non-stick and/or non-wetting coating can be reduced.

Preferably, a surface roughness is less than Ra 10.00 μm, preferably less than Ra 7.50 μm, more preferably less then Ra 5.00 μm.

A higher surface roughness—within the limits mentioned above—may be advantageous as this would improve adhesive capacity for the non-stick and/or non-wetting coating.

Preferably, a surface roughness is more than Ra 0.01 μm, preferably more than Ra 0.10 μm, more preferably more than Ra 0.20 μm, still more preferably more than Ra 0.50 μm, still more preferably more than Ra 1.00 μm, still more preferably more than Ra 2.00 μm.

The person skilled in the art knows various standard methods to determine the surface roughness value Ra. Particularly the surface roughness may be preferably determined in accordance with BS EN ISO 4287:2000 British standard, identical with the ISO 4287:1997 standard.

In a preferred embodiment, the ceramic non-stick and/or non-wetting coating comprises at least a first and a second layer, wherein the first layer is applied to the pretreated inner surface of the fan cover or the surface of the ground layer or the pretreated surface of the protective layer, and wherein the second layer is applied to the first layer, so that a fan cover comprising a coating having base layer and top layer is obtained.

Particularly, the second layer is obtained by a sol-gel process from a second composition comprising a silica sol, a silane and a siloxane, in particular a polydimethylsiloxane. Such a coating having at least a second layer is also known from EP 2 177 580 B1.

In other words: The second layer, hereinafter also referred to as top layer, is composed of a matrix comprising the condensation reaction product of a silica sol, a silane and a siloxane. Obtained by a sol-gel process from a second composition has therefore to be understood in such a way, that the second layer is obtained by means of hydrolysis and (poly-)condensation of an aqueous mixture comprising at least a silica sol, a silane and a siloxane. Thus, the starting materials of the second composition, namely at least silica sol, silane, siloxane and water if needed, are mixed together and stirred. The mixture obtained thereof is then applied to the inner surface of the first layer and dried to obtain the second layer.

Preferably, the silica sol is present in an amount of 15 to 70 wt %, in particular in an amount of 30 to 70 wt % and/or the silane is present in an amount of 2 to 70 wt %, in particular in an amount of 10 to 40 wt %, both in the first and/or the second composition and/or in the first and/or second layer.

In particular the silane in the first and/or the second composition and/or in the first and/or second layer is an organoalkoxysilane, in particular a methyltrimethyoxysilane and/or a fluoralkoxysilane.

The first and/or the second composition and/or in the first and/or second layer may comprise a catalyst, in particular an acidic catalyst, more preferably an organic compound containing one or more carboxyl groups and/or a mineral acid, e.g. hydrochloric acid, sulfuric acid or nitric acid. Different monocarboxylic or dicarboxylic acids like formic acid, acetic acid or oxalic acid may be mentioned here as an example for suitable catalysts.

It has been shown, that it is advantageous, if the catalyst is present in an amount of 0.1 to 5 wt %, in particular in an amount of 0.1 to 2 wt %, both in the first and the second composition and/or in the first and/or second layer. The catalyst generally acts as a catalyst in the hydrolysis and condensation reaction and prevents too slow crosslinking.

In another preferred embodiment, the first and/or the second composition and/or in the first and/or second layer comprise or comprises a solvent, in particular an organic solvent. Examples for organic solvents are alcoholic solvents, for example methanol, ethanol or propanol.

In particular, the solvent is present in an amount of 10 to 60 wt %, in particular in an amount of 10 to 40 wt %.

Like the second composition or second layer, the first composition or first layer may also comprise a siloxane in a preferred embodiment. Said siloxane is in particular a polydimethylsiloxane.

An advantageous amount of siloxane in the first composition or first layer as well as in the second composition or second layer is between 0.1 to 2 wt %.

In a preferred embodiment, the first composition or first layer and/or the second composition or second layer comprise or comprises pigments and/or dyes and/or filling materials and/or further additives. In particular, temperature resistant an organic pigments are favourable.

In addition, water is added to the first composition and/or the second composition, if needed.

The second object of the present invention is achieved by a cooking appliance according to claim 17.

Such a cooking appliance comprises a heatable cavity (or: oven muffle) comprising at least one cavity wall (or: muffle wall) defining a cooking chamber for cooking foodstuff, a central opening for placing foodstuff into the cooking chamber, heating elements for heating said cavity, and a door for closing the cavity, in particular for closing the central opening of the cavity, and a fan. The cooking appliance further comprises a fan cover according to the present invention that is in particular attached to a rear cavity wall of the heatable cavity.

For example, a cooking appliance according to the present invention is a cooking and/or baking device for cooking and/or baking of foodstuff. Such cooking appliance, preferably a cooking and/or baking device, may particularly be a cooking appliance selected from the group comprising an oven, baking oven, microwave, steam-oven, and steam-cooker.

The cooking appliance may comprise further components that are provided to be attached to the cavity walls, for example baking trays or grids. Even these further components may have a comparable structure to the fan cover, thus at least one surface of such accessory components may be pretreated by one of the methods mentioned above and coated with a ceramic non-stick and/or non-wetting coating comprising at least a first layer, wherein the first layer is obtained by a sol-gel process from a first composition comprising a silica sol and silane.

The components of the cavity, thus the cavity walls and/or further accessory components, may be integrally formed or may comprise several parts that are provided to be joined together to form a shaped structure, such as the heatable oven cavity, its front frame or any component thereof.

Heating elements, e.g. tube heating elements or grill elements, can heat up the cavity inner surface up to 500° C. For the use of a non-stick and non-wetting coating, in particular a coating obtained by a sol-gel process being described herein, that is easy to clean, the maximum temperature needs to be below about 300 to 350° C.

The third object of the present invention is achieved by a method for manufacturing a fan cover according to claim 18.

Such a method for manufacturing a fan cover having a non-stick and/or non-wetting coating on an inner surface comprises at least the following steps:

Firstly, a fan cover having an inner surface, an outer surface and at least one opening allowing a fluid to flow through is provided.

Secondly, at least the inner surface of the fan cover is pretreated by mechanical treatment, physical treatment or chemical treatment, in particular by sandblasting and/or laser treatment and/or an enamelling process to form a ground layer to provide a surface having a roughness being suitable for applying a non-stick and/or non-wetting coating.

In other words: Before applying the non-stick and/or non-wetting coating or a protective layer to the fan cover, the surface of the fan cover is prepared, e.g. by cleaning and/or drying the inner surface of the fan cover and/or activating the inner surface, preferably by roughening the surface by sandblasting and/or laser treatment and/or an enamelling process to form a ground layer on the inner surface of the fan cover, in order to achieve better adhesion of the protective layer or the non-stick and/or non-wetting coating.

Regarding the pretreatment and roughening of the surface, we explicitly refer to the explanations and examples described with regard to the different methods and materials used for the different layers, in particular the different enamel compositions, described in the context of the protective and/or ground layer as well as composition of the non-stick and/or non-wetting coating described in the context of the fan cover.

Thirdly, at least one layer of the non-stick and/or non-wetting coating is applied to the pretreated inner surface of the fan cover or the surface of the ground layer.

According to a preferred embodiment, during the enameling process of the pretreatment, an enamel powder is applied onto the inner surface of the fan cover and/or onto the surface of the protective layer, and the enamel powder is heated for providing a ground layer with an inner surface having a roughness being suitable for applying the non-stick and/or non-wetting coating.

According to a preferred embodiment, a protective layer, in particular an enamel layer and/or an aluminium layer and/or a layer comprising aluminium, is applied to at least the inner surface of the fan cover and the surface of the protective layer is pretreated by sandblasting and/or laser treatment and/or an enamelling process to form a ground layer.

In other words: Before applying the non-stick and/or non-wetting coating to the surface of the protective layer, the surface of the protective layer is prepared, e.g. by cleaning and/or drying the inner surface and/or activating the inner surface, preferably by roughening the surface for example by mechanical treatment, in particular sandblasting, laser treatment or chemical treatment of the inner surface of the fan cover, in order to achieve better adhesion of the non-stick and/or non-wetting coating. Also preferred is even here, that the surface of the protective layer is made and/or roughened by an enameling process, in particular by applying a ground layer.

Thus the ground layer is applied to the inner surface of the at least one fan cover by an enamelling process having a surface roughness between Ra 0.01 μm to 10.00 μm, more preferably between Ra 0.10 μm to 5.00 μm, still more preferably between Ra 0.20 μm to 5.00 μm, still more preferably between Ra 0.50 μm to 5.00 μm, still more preferably between Ra 2.00 μm to 5.00 μm, still more preferably between Ra 2.50 μm to 5.00 μm and/or a layer thickness between 5 and 100 μm, in particular between 10 and 80 μm, more particularly between 20 and 50 μm. Said properties can be achieved by using enamel compositions described before, thus conventional enamels, catalytic enamels or so-called ground compositions. We refer explicitely to the explanations and examples described in the context of the protective and ground layer of the fan cover.

In a preferred embodiment, the non-stick and/or non-wetting coating comprises a first and a second layer, wherein the first layer is applied to the inner surface of the fan cover or the surface of the ground layer or the pretreated surface of the protective layer, and wherein the second layer is applied to the first layer, so that a fan cover comprising a coating having base layer and top layer is obtained.

Novel and inventive features of the present invention are set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in further detail with reference to the drawings, in which

FIG. 1 illustrates a schematic view of a cooking appliance including a fan cover according to an embodiment of the present invention,

FIG. 2 illustrates a fan cover according to one embodiment in a perspective view,

FIG. 3 illustrates a cross-sectional view of the fan cover according to a first embodiment of the invention in detail,

FIG. 4 illustrates a cross-sectional view of the fan cover according to a second embodiment of the invention in detail,

FIG. 5 illustrates a cross-sectional view of the coating according to an embodiment of the invention in detail.

DETAILED DESCRIPTION

FIG. 1 illustrates a schematic view of a cooking appliance 1. Such cooking appliance 1 comprises a heatable cavity 2 having cavity walls 3 which define a cooking chamber 4 into which foodstuff may be placed for cooking or baking. The cooking chamber 4 is defined by the cavity walls 3, usually comprising a left and a right side wall, a bottom wall and an upper wall as well as a rear wall and a front wall. One of the cavity walls 3, usually the front wall (not shown in FIG. 1 ) comprises a central opening for placing the foodstuff into the cooking chamber 4. The central opening may be closed or opened by a door (not shown in FIG. 1 ) that may be a part of the front wall. The cooking appliance 1 usually further comprises also heating elements 5 presently disposed at the upper cavity wall of the cavity 2 for heating the cooking chamber 4 and therefore heating food that has been placed therein to be cooked.

The cooking appliance 1 further comprises a fan cover 6 being disposed at the rear cavity wall of the cavity 2 in front of a fan 9 of the cooking appliance 1. The fan cover 6 is essentially plate shaped and might for example be attached to the rear cavity wall by screws. The fan cover 6 can be made of a corrosive material, in particular corrosive steel, or an non-corrosive material, in particular stainless steel. An exemplary embodiment of a fan cover 6 is shown in FIG. 2 . The fan cover 6 comprises an inner surface 7 a, an outer surface 7 b and openings 8 for allowing a fluid to flow through. Of course, the fan cover 6 can have other shapes or openings of different design than shown in FIG. 2 and can be attached to the rear cavity wall by alternative well-known mounting techniques.

The cooking appliance 1 further comprises a control panel 10 with several operating and display elements, e.g. temperature display or oven setting knobs. Furthermore, mounting grids and shelves 11 are disposed at a left and a right cavity wall 3.

FIG. 3 shows a cross-sectional view of the fan cover 6 according to a first embodiment in which the layer structure can be seen. The fan cover 6 is obtained by a process wherein the inner surface 7 a of the fan cover 6 is pretreated by an enamelling process to form a ground layer 13 formed by a ground coat composition that has burned to the fan cover 6 at temperatures of about 820° C. on said surface 7 a. Said ground layer 13 or, respectively, enamel layer, has an inner surface 14 a and an outer surface 14 b and shows a surface roughness Ra of about 2 to 3.5 μm at its inner surface 14 a. The thickness of the ground layer 13 is about 20 μm. Ground layer 13 also shows corrosion resistant properties so that the fan cover 6 is protected against corrosion, what is in particular useful and necessary if said fan cover 6 is made of a corrosive material, for example a low carbon steel to save costs. Inventors have surprisingly found that such a small thickness and small roughness shows good adhesion properties that are sufficient for applying a non-stick and/or non-wetting coating 12 comprising at least one layer 17 that is obtained by a sol-gel process from a first composition comprising a silica sol and a silane and according to embodiments of the invention further components like solvents or additives in useful amounts as described before.

FIG. 4 shows a cross-sectional view of the fan cover 6 according to a second embodiment in which the layer structure can be seen. The fan cover 6 is presently made from a corrosive material so that a protective layer 15 is applied to the inner surface 7 a of the fan cover 6 before the pretreatment step for preventing the fan cover 6 from corrosion, the protective layer 15 having an inner surface 16 a and an outer surface 16 b. Afterwards, for obtaining a fan cover 6 with a non-stick and/or non-wetting coating 12, said protective layer 15 is pretreated by sandblasting or laser treatment to achieve a surface roughness of the surface 16 a of the protective layer between about 2 and 3.5 μm by sandblasting or a surface roughness smaller than fpm by laser treatment. Inventors have surprisingly found out that even though the surface roughness achieved by laser treatment is smaller, good adhesion behaviour can be achieved, probably due to more fine-grained structure in comparison with a sandblasted surface. The protection layer 15 has a layer thickness of about 150 μm. In particular in case that the protection layer 15 is an aluminium layer or a layer comprising aluminium, it might be useful to carry out an enamelling process to form a ground coat 13 on the surface 16 a of the protective layer 15 (shown in dashed lines) to benefit from the good adhesive properties of an enamel layer made from a ground composition having a surface roughness between 2 to 3.5 μm.

FIG. 5 shows a cross-sectional view of the fan cover 6 according to a third embodiment, that differs from the first embodiment according to FIG. 3 in that the non-stick and/or non-wetting coating 12 comprises a first and a second layer 17, 18, wherein the first layer 17 is applied to the surface 14 a of the ground layer 13 and wherein the second layer 18 is applied to the first layer 17. The second layer 18 is obtained by a sol-gel process from a second composition comprising a silica sol, a silane and a siloxane.

As a matter of course, even a fan cover 6 according to the second embodiment shown in FIG. 4 might have a non-stick and/or non-wetting coating 12 comprising a first and a second layer 17, 18, wherein the first layer 17 is applied to the surface 16 a of the protective layer 15 and wherein the second layer 18 is applied to the first layer 17.

Both, the first layer 17 and, if present, the second layer 18 are obtained by a sol-gel process. For production of the first layer 17, a colloidal silica sol, which is pure SiO₂, an organoalkoxysilane, which is an organic-inorganic hybrid material and presently methyltrimethyoxysilane, and an acid catalyst, presently acetic acid are mixed and stirred for about 2 hours at room temperature in order to effect a condensation reaction. The weight proportion of organoalkoxysilane is about 10 to 40 wt %, silica sol is present in an amount of 30 to 70 wt %. Adding acetic acid in an amount of 0.1 to 2 wt % is sufficient for accelerating the condensation reaction.

Then, a solvent, presently propanol, is added in an amount of 10 to 40 wt %. Pigments are added and further additives can be additionally added at this step.

For production of the second layer 18, colloidal silica sol, siloxane and a solvent, even here propanol, are mixed. Organoalkoxysilane and an acid catalyst, presently acetic acid are then added. Even said mixture is mixed and stirred for about 2 hours at room temperature. The weight proportion of organoalkoxysilane is about 10 to 40 wt %, silica sol is present in an amount of 30 to 70 wt %, acetic acid is present in an amount of 0.1 to 2 wt %. Siloxane, presently polydimethylsiloxane, is added in an amount of 0.1 to 2 wt %.

Then, a solvent, presently even here propanol, is added in an amount of 10 to 40 wt %. Pigments are added and further additives can be additionally added at this step.

For manufacturing a fan cover 6 having a non-stick and/or non-wetting coating 12 on its inner surface 7 a, a fan cover 6 having an inner surface 7 a, an outer surface 7 b and at least one opening 8 allowing a fluid to flow through is provided. In a further step, the inner surface 7 a of the fan cover 6 is pretreated in order to provide good adhesion properties and surface roughness for subsequent apply of the non-stick and/or non-wetting coating (FIG. 3 ). Additionally or in an alternative, a protective layer 15 is applied to the inner surface 7 a of the fan cover 6 and said protective layer 15 is pretreated (FIG. 4 ). Pretreatment of the surface 7 a of the fan cover 6 or, if present, the surface 16 a of the protective layer 15 is carried out in particular by sandblasting, laser treatment or an enamelling process for forming a ground layer 13.

The enamelling process is in particular carried out by applying an enamel powder onto the inner surface 7 a of the fan cover 6 and/or onto the surface 16 a of the protective layer 15, and heating the enamel powder for providing a ground layer 13 with a surface 14 a having a roughness being suitable for applying the non-stick and/or non-wetting coating 12.

Afterwards, at least one layer of the non-stick and/or non-wetting coating 12 is applied to the inner surface 7 a of the fan cover 6 or, if present, to the inner surface 16 a of the protective layer 15 or the surface 14 a of the ground layer 13.

In case of applying a non-stick and/or non-wetting coating 12 comprising at least two layers 17, 18 (FIG. 5 ), the first layer 17 is applied to the inner surface 7 a of the fan cover 6 or the surface 14 a of the ground layer 13 or the surface 16 a of the protective layer 15, and the second layer 18 is applied to the first layer 17. Both layers 17, 18 have an outer surface 17 a, 18 a facing away from the fan cover 6 and an inner surface 17 b, 18 b facing towards the fan cover 6.

The present invention, thus a fan cover 6 having a non-stick and/or non-wetting coating 12 and a cooking appliance 1 having such a fan cover 6 shows an improved cleaning behaviour. The coating 12 is abrasion-resistant and shows a greater surface hardness what leads to an increased lifetime. Furthermore, the coating 12 has shown a good adhesion properties on the fan cover 6, in particular on the surface 14 a of the ground layer 13 or the surface 16 a of the protective layer 15. As the coating 12 is produced by a sol-gel process, a (second) burning step at high temperatures is not necessary.

Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the present invention is not limited to those precise embodiments, and that various other changes and modifications may be affected therein by one skilled in the art without departing from the scope or spirit of the invention. All such changes and modifications are intended to be included within the scope of the invention as defined by the appended claims.

LIST OF REFERENCE NUMERALS

-   1 cooking appliance -   2 cavity -   3 cavity wall -   4 cooking chamber -   5 heating element -   6 fan cover -   7 a inner surface of the fan cover -   7 b outer surface of the fan cover -   8 openings -   9 fan -   10 control panel -   11 mounting grids and shelves -   12 non-stick and/or non-wetting coating -   13 ground layer -   14 a inner surface of the ground layer -   14 b outer surface of the ground layer -   15 protective layer -   16 a inner surface of the protective layer -   16 b outer surface of the protective layer -   17 first layer of the non-stick and/or non-wetting coating -   17 a inner surface of the first layer -   17 b outer surface of the first layer -   18 second layer of the non-stick and/or non-wetting coating -   18 a inner surface of the second layer -   18 b outer surface of the second layer 

1. A fan cover for a cooking appliance, produced by a process comprising the following steps: a) providing a fan cover having an inner surface, an outer surface and at least one opening allowing a fluid to flow through, b) optionally pretreating at least the inner surface of the fan cover to provide a surface roughness suitable for applying a non-stick and/or non-wetting coating by mechanical treatment, physical treatment or chemical treatment, to form a ground layer, c) applying the non-stick and/or non-wetting coating to the inner surface of the fan cover or a surface of the ground layer, wherein the non-stick and/or non-wetting coating comprises at least one layer that is obtained by a sol-gel process from a first composition comprising a silica sol and a silane.
 2. The fan cover according to claim 1, wherein the fan cover is made of a corrosive material, or an non-corrosive material.
 3. The fan cover according to claim 1, wherein the fan cover is provided with a protective layer that is applied to at least the inner surface of the fan cover, said protective layer is pretreated by sandblasting and/or laser treatment and/or an enamelling process for providing a surface having a roughness suitable for applying the non-stick and/or non-wetting coating, and the at least one layer of the non-stick and/or non-wetting coating is applied to the surface of said protective layer.
 4. The fan cover according to claim 3, wherein the protective layer is an enamel layer and/or an aluminium layer and/or a layer comprising aluminium.
 5. The fan cover according to claim 3, wherein the protective layer has a thickness between 50 and 250 μm.
 6. The fan cover according to claim 1, wherein the ground layer, is an enamel layer obtained by the enamelling process and has a layer thickness between 10 and 80 μm.
 7. The fan cover according to any one of the preceding claim 1, wherein the inner surface of the fan cover and/or said surface of the ground layer and/or said surface the protective layer has a surface roughness between Ra 0.01 μm to 10.00 μm.
 8. The fan cover according to claim 1, wherein the non-stick and/or non-wetting coating comprises at least a first layer and a second layer, wherein the first layer of the non-stick and/or non-wetting coating is applied to the inner surface of the fan cover or to the surface of the ground layer or to the surface of the protective layer, and wherein the second layer of the non-stick and/or non-wetting coating is applied to the first layer of the non-stick and/or non-wetting coating.
 9. The fan cover according to claim 8, wherein the second layer of the non-stick and/or non-wetting coating is obtained by a sol-gel process from a second composition comprising a silica sol, a silane and a siloxane.
 10. The fan cover according to claim 1, wherein the silica sol is present in an amount of 15 to 70 wt % and/or the silane is present in an amount of 2 to 70 wt %, within the first composition.
 11. The fan cover according to claim 1, wherein the silane is an organoalkoxysilane and/or a fluoralkoxysilane.
 12. The fan cover according to claim 1, wherein the first composition comprises an organic solvent, and/or an acidic catalyst.
 13. The fan cover according to claim 12, wherein the solvent is present in said first composition in an amount of 10 to 60 wt %, and/or the catalyst is present in said first composition in an amount of 0.1 to 5 wt %.
 14. The fan cover according to claim 1, wherein the first composition comprises a siloxane.
 15. The fan cover according to claim 9, wherein the siloxane is present in said second composition in an amount of 0.1 to 2 wt %.
 16. The fan cover according to claim 1, wherein the first composition comprises pigments and/or dyes and/or filling materials and/or further additives.
 17. A cooking appliance comprising: a heatable cavity, comprising at least one cavity wall defining a cooking chamber for cooking foodstuff and a central opening for placing foodstuff into the cooking chamber, a heating elements for heating said cavity, and a door for closing the cavity, in particular for closing the central opening of the cavity, a fan, and the fan cover according to claim 1 attached to the at least one cavity wall.
 18. A method for manufacturing a fan cover of a cooking appliance, having a non-stick and/or non-wetting coating on an inner surface of the fan cover, comprising the following steps: providing a fan cover having an inner surface, an outer surface and at least one opening allowing a fluid to flow through, pretreating at least the inner surface of the fan cover by mechanical treatment, physical treatment or chemical treatment to form a first ground layer, applying at least one layer of the non-stick and/or non-wetting coating to the inner surface of the fan cover or to a surface of said first ground layer.
 19. A method for manufacturing a fan cover according to claim 18, further comprising applying a protective layer comprising enamel and/or aluminium to at least the inner surface of the fan cover and pretreating a surface of the protective layer by sandblasting and/or laser treatment and/or an enamelling process to form a second ground layer.
 20. A method for manufacturing a fan cover according to claim 19, further comprising applying an enamel powder onto the inner surface of the fan cover and/or onto the surface of the protective layer, and heating the enamel powder to provide said first ground layer and/or said second ground layer with a roughness suitable for applying the non-stick and/or non-wetting coating.
 21. A method for manufacturing a fan cover according to claim 19, wherein the non-stick and/or non-wetting coating comprises a first layer and a second layer, wherein the first layer of the non-stick and/or non-wetting coating is applied to the inner surface of the fan cover or to the surface of the first ground layer or to the surface of the protective layer, and wherein the second layer of the non-stick and/or non-wetting coating is applied to the first layer of the non-stick and/or non-wetting coating.
 22. A coated fan cover for a cooking appliance, comprising: a fan cover made of low-carbon steel and having a fan-cover surface, said fan-cover surface being roughened by sandblasting or by laser treatment to achieve a first surface roughness of Ra 0.01 μm to 10.00 μm according to ISO 4287:1997; a protective layer applied over the roughened fan-cover surface, said protective layer comprising a non-corrosive material selected from enamel and aluminum and having a thickness between 50 μm and 250 μm, said protective layer having an outer surface facing away from said fan cover; a ground layer comprising enamel applied to said outer surface of said protective layer, the ground layer having an outer surface facing away from said protective layer that has a second surface roughness up to about 3.5 μm according to ISO 4287:1997 provided by sand-blasting or by laser treatment of the outer surface of said ground layer; a first non-stick, non-wetting solgel coating layer deposited on the roughened outer surface of said ground layer, said first non-stick, non-wetting coating layer having been deposited as a first composition comprising 30 wt. % to 70 wt. % silica sol and 10 wt. % to 40 wt. % silane that underwent a first condensation reaction therebetween to yield the first solgel coating layer; and a second non-stick, non-wetting solgel coating layer deposited on an outer surface of said first non-stick, non-wetting coating layer, said second non-stick, non-wetting coating layer having been deposited as a second composition comprising 30 wt. % to 70 wt. % silica sol, 10 wt. % to 40 wt. % silane, and 0.1 wt. % to 2 wt. % siloxane that underwent a second condensation reaction therebetween to yield the second solgel coating layer; said second solgel coating layer having an outer surface configured to face a cooking cavity of a cooking appliance in-use whose water-contact angel is at least 95°. 